Is there a way to know name clashes in PostScript level between ones used in PSTricks packages and our own definitions?

Question

\documentclass[pstricks,border=12pt]{standalone}
\usepackage{pstricks-add}

\begin{document}

\begin{pspicture}[showgrid=bottom](4,2)
\pstVerb
{
    /a 2 def
    /bb 1 def
}%
\rput(!a bb){First Page}
\end{pspicture}

\end{document}

My a silently overrides the PSTricks' definition. I need a way to know this silent substitution. Is there a way to know name clashes in PostScript level between ones used in PSTricks packages and our own definitions?

Answer 1

I suppose the correct thing to do is always make your local definitions in a private dictionary to avoid the problem but this postscript addresses the question as asked:

The sequence

 dup where {pop == (already defined) ==} {3 def} ifelse

will define the preceding token to 3 if it is not defined, and leave it untouched and echo a warning if it is defined.

tested with /a defined to be 2 and /b undefined

GS>/a 2 def
GS>/a dup where {pop == (already defined) ==} {3 def} ifelse
/a
(already defined)
GS>a ==
2
GS>/b dup where {pop == (already defined) ==} {3 def} ifelse
GS> b ==
3

a gets left as 2 but b is defined to be 3.

Answer 2

\documentclass{article}
\usepackage{pstricks}\SpecialCoor
\begin{document}

\begin{pspicture}[showgrid=bottom](4,2)
\pstVerb
{  userdict begin
    /a 2 def
    /bb 1 def
  end }
\rput(! userdict begin a bb end){First Page}
\end{pspicture}

\end{document}